Method for producing peroxodisulfates in aqueous solution

ABSTRACT

A process for preparing or regenerating peroxodisulfuric acid and its salts by electrolysis of an aqueous solution containing sulfuric acid and/or metal sulfates at diamond-coated electrodes without addition of promoters is described, with bipolar silicon electrodes which are coated with diamond on one side and whose uncoated silicon rear side serves as cathode being used.

The invention relates to a process for preparing or regeneratingperoxodisulfuric acid and its salts by electrolysis of an aqueoussolution containing sulfuric acid and/or metal sulfates. As used herein,the term “metal sulfates” encompasses both sulfates of metals such aszinc, nickel or iron and sulfates of alkali metals and alkaline earthmetals and also ammonium sulfate. Thus, it is possible to use, forexample, alkali metal sulfates or alkaline earth metal sulfates,preferably alkali metal sulfates or ammonium sulfate, as metal sulfates.It is also possible to use mixtures of various metal sulfates, forexample magnesium sulfate, zinc sulfate or else nickel or iron sulfate,preferably in the regeneration of etching and pickling solutions.

It is known from the prior art that diamond-coated electrodes composedof valve metals, preferably niobium, or ceramic materials, preferablysilicon, can be used for the preparation of peroxodisulfates of thealkali metals and of ammonium [DE 199 48 184.9, DE 100 19 683]. Thediamond layer is made conductive by doping with a trivalent orpentavalent element, preferably boron. These have advantages over thesmooth platinum anodes which have hitherto been exclusively used inperoxodisulfate production in that, as a result of the high potentialwhich can be achieved on the diamond surface, it is not necessary to addpotential-increasing additives to the electrolyte in order to achievesufficiently high current yields, as is unavoidable in the case ofplatinum anodes. The preferred use of thiocyanates as polarizers resultsin anode gases which contain cyanide and make complicated gaspurification measures necessary. When diamond-coated anodes are used,these can be dispensed with.

A further advantage of diamond-coated anodes in peroxodisulfateproduction is that, even at a low sulfate content in the anolyte,significantly higher current yields can be achieved than when usingplatinum anodes.

However, despite the good stability of, in particular, diamond-coatedsilicon electrodes, their use is associated with a number ofdisadvantages. Thus, there is the problem of suitable supply of electriccurrent. Owing to the relatively low electrical conductivity of thesilicon base body, a contact has to be provided over the entire area ofthe reverse side of the electrode, so that current needs to flow onlyfrom the contacted rear side through the small thickness of the siliconelectrode of about 1-2 mm to the diamond coating. Although this problemcould in principle be solved by adhesive bonding of the preferablymetallized rear sides of the silicon plates to a metallic substratehaving a good conductivity by means of an electrically conductiveadhesive, this is relatively complicated.

A further disadvantage of the diamond-coated silicon electrodes of theprior art is their limited dimensions of at present not more than200×250 mm. In order to nevertheless be able to provide large-areaanodes for use in industrial electrolysis cells, EP 1 229 149 proposedadhesively bonding a relatively large number of such silicon-diamondelectrodes by means of an electrically conductive adhesive to a metalbase plate, e.g. composed of a valve metal, and sealing the edges bymeans of a corrosion-resistant resin, e.g. epoxy resin. However, thedifficulties involved, for example in. the provision of the conductiveadhesive, e.g. an adhesive composed of epoxy resin containing silverparticles, and in the complete elimination of the oxide layers on theareas to be joined, are very great. In addition, such an electrodeconstruction has been found to be insufficiently corrosion resistant forthe preparation of peroxodisulfate, so that only short operation livesof usually less than one year can be achieved in this way.

Another possible way disclosed in the prior art for constructingelectrolysis cells having a sufficiently large current capacity is toconnect a relatively large number of bipolar silicon-diamond electrodesin series. FR 2790268 B1 discloses such a bipolar electrolysis cell inwhich the bipolar electrodes comprise a ceramic substrate which iscompletely enveloped by a diamond film. However, this cell is notproposed specifically for the preparation of peroxodisulfates but foruses in the degradation of pollutants or for disinfection of water.

DE 200 05 681 describes the use of bipolar electrodes coated on bothsides with diamond layers.

EP 1 254 972 proposes an electrolysis cell construction which issuitable for various applications and can be configured as a monopolaror bipolar, undivided or divided cell. In the bipolar design, silicondisk electrodes coated on both sides with a diamond layer are once againexclusively used. In the preparation of peroxodisulfates, these cellshaving silicon electrodes coated on both sides with a diamond layer andthe relatively complicated cell construction can be used effectivelyonly for small persulfate throughputs. If an attempt is made to increasethe throughput to industrially relevant ranges by means of a relativelylarge number of individual bipolar cells, this construction results inreduced yields due to the loss currents in the power supply leads andpower outlet leads which increase greatly with the total voltage.

It was therefore an object of the present invention to provide a processfor preparing or regenerating peroxodisulfuric acid and/or its salts, inwhich the above-described disadvantages of previous processes andelectrolysis cells are at least partly avoided. It has been found thatperoxodisulfates can advantageously be prepared in undivided or dividedelectrolysis cells in a simple manner by using bipolar siliconelectrodes which have been coated on one side with doped diamond, withthe uncoated silicon rear sides acting directly as cathodes.

According to the invention, the coating on the silicon electrode has athickness of from about 1 to about 20 μm, preferably about 5 μm.

It was highly surprising that only the coating on the anode side of thebipolar electrode is necessary in order to achieve satisfactory resultswith the uncoated silicon rear side which then functions as cathode. Inthe case of an undivided bipolar cell, it was also surprisingly foundthat lower persulfate losses occur as a result of cathodic reductionwhen using a silicon cathode according to the invention compared to themetal cathodes which are usually used in the prior art in persulfateproduction.

Furthermore, it has been found that it is not only possible to achievehigh persulfate formation rates when using the bipolar electrodesaccording to the invention but this can be achieved even at very lowcell voltages and thus low specific electric energy consumptions. Thisis based firstly on the recognition that the silicon cathode surfacesare freed of the poorly conductive oxide layers which are initiallypresent by means of the cathodic reaction and are also kept completelyfree during the course of the electrolysis. For example, it was found ina long-term experiment (cf. example 1) that the cell voltage is evenreduced further with increasing time of operation, while in the case ofthe diamond-coated silicon electrodes adhesively bonded to a metalsubstrate according to the prior art, an opposite tendency is observedas a result of increasing corrosion.

The process of the invention thus advantageously makes it possible toprepare peroxodisulfuric acid and/or its salts at a genuine bipolarelectrode with a high current yield and a low electric energyconsumption even though only the slightly conductive silicon is used ascathode. In addition, no costs for a cathode coating are incurred.

A further advantage of the inventive bipolar silicon electrodes coatedon one side with diamond is the lower catalytic activity of the siliconrear side compared to a metallized electrode rear side, e.g. composed ofplatinum or stainless steel. It has been found that reduction losses ofperoxodisulfate are therefore lower when electrolysis is carried out inan undivided electrolysis cell. This leads, in the case of undividedcells, to the increase in the peroxodisulfate concentration withelectrolysis time being somewhat steeper and the achievable finalconcentration being higher than when a metallized cathode is used underotherwise identical electrolysis conditions.

Compared to the bipolar electrodes of the prior art which are coatedwith doped diamond on both sides, cost savings are advantageouslyachieved both for the electrodes themselves and for the electrolysiscells equipped therewith and also as a result of the lower electricenergy consumptions which can be achieved.

The process of the invention for preparing peroxodisulfuric acid and/orits salts can be carried out both in undivided electrolysis cells and inelectrolysis cells which are divided, for example by means ofion-exchange membranes or porous diaphragms.

The bipolar silicon electrodes according to the invention which arecoated on one side with diamond are particularly useful for undividedelectrolysis cells having a relatively simple construction, as aredescribed, for example, in DE G 200 05 681.6 for the disinfection ofwater. It is advantageous in terms of the current input for themonopolar boundary anodes to comprise a diamond-coated valve metal. Theterm “valve metal” refers to a metal which when connected as an anodebecomes coated with an oxide layer which becomes nonconductive even athigh voltages. Connected as anode, the metal blocks. Connected ascathode, the oxide layer is dissolved and current flows in a fairlyuninhibited fashion. Thus, valve metals behave like a rectifier whendifferent polarities are applied. Examples of suitable valve metals aretantalum, titanium, niobium and zirconium. For the purposes of thepresent invention, preference is given to using niobium.

The monopolar boundary cathodes preferably comprise a suitable materialhaving a good conductivity, e.g. stainless steel, Hastelloy, platinumand impregnated graphite. For the purposes of the present invention,preference is given to using high-alloy stainless steels or Hastelloy. Asilicon boundary cathode having a metallized rear side and with acurrent supply plate composed of a material having a good conductivity,e.g. copper, as contact can also be used due to the good long-termstability in undivided cells. Particularly when using boundaryelectrodes composed of metallic materials, optimal current input can beachieved in a simple manner and without large voltage drops because ofthe good conductivity.

It is also possible for a plurality of electrode stacks comprisingbipolar electrodes and boundary electrodes with power supply lead to beconnected electrically in parallel in an electrolysis cell. Ifnecessary, the spacing between the bipolar electrodes can be set orfixed by means of spacers. Such electrode stacks connected in parallelmake it possible to accommodate relatively large power capacities in anelectrolysis cell without an unjustifiably high total voltage beingnecessary. The voltage can thus also be optimally matched to theavailable rectifier voltage. In addition, the short circuit currents inthe common feed and discharge lines for the electrolyte solutions can beminimized further as a result, which can additionally be aided in aknown manner by installation of additional resistance sections in theselines.

Undivided bipolar cells having the structure provided by the inventioncan be used particularly advantageously when the peroxodisulfateconcentration does not have to be very high for the application inquestion, for example for the oxidative degradation of pollutants inprocess solutions and wastewater. As can be seen from example 2, sodiumperoxodisulfate reaction solutions having a content of from 50 to 100g/l can be prepared very effectively in batch operation in an undividedcell provided with the bipolar electrodes according to the invention atcurrent yields of from 75 to 50% and specific electric energyconsumptions of from 1.3 to 1.9 kWh/kg.

Even better current yields or the same yields at higher finalperoxodisulfate concentrations can be achieved by shielding of thecathode by means of suitable materials which inhibit mass transfer tothe cathode surface, as can be seen from example 3. Materials suitablefor these purposes are, for example, PVC gauzes. The process of theinvention thus makes it possible to obtain sodium peroxodisulfateconcentrations from 150 to 200 g/l with justifiable current yields ofabout 50% in undivided cells, albeit at relatively high cell voltages.

If higher final concentrations of peroxodisulfates, e.g. in the rangefrom 200 to 400 g/l of sodium peroxodisulfate, are desired, the use ofdivided electrolysis cells provided with the bipolar silicon electrodesaccording to the invention is preferred. As can be seen from example 4,current yields of from about 75 to 85% can be achieved in this way,albeit with a more complicated cell construction and higher cellvoltages of from about 5.5 to 6 V. However, comparatively very goodspecific electric energy consumptions of less than 2.0 kWh/kg can stillbe achieved in this way.

A further surprising effect of the process of the invention are the verylow corrosion rates at the silicon cathodes which are found in undividedelectrolysis cells in a long-term experiment using an acidicpersulfate-containing electrolyte. Thus, surprisingly low corrosionrates of only 2-3 μm were found in an undivided cell at a steady-statesodium peroxodisulfate content of about 150 g/l in a long-termexperiment over about 7 months (cf. example 1). This was particularlysurprising because 10-100 times greater corrosion was observed even onplatinum cathodes of the prior art under these very highly corrosiveconditions. Even cathodes made of graphite or high-alloy stainlesssteels were found to be unsuitable in such peroxodisulfate-containingsulfuric acid electrolyte solutions because they were insufficientlycorrosion-resistant.

EXAMPLES Example 1

An undivided bipolar electrolysis cell having a construction analogousto that in DE G 200 05 681.6 contained 9 bipolar silicon electrodescoated on one side with about 3 μm of boron-doped diamond (average about3000 ppm of boron). A niobium electrode coated on one side with diamondand provided with a power supply lead served as boundary anode. Theboundary cathode with power supply lead comprised Hastelloy. The bipolarelectrodes had a dimension of 100×33 mm (33 cm²) . The mean spacing ofthe about 1 mm thick bipolar electrodes was set to about 2 mm by meansof spacers. The electrolysis current was regulated at a constant 16.5 A,corresponding to an anodic and cathodic current density of 0.5 A/cm².The total current capacity of the electrolysis cell was thus 10×16.5=165A. 2 l of an aqueous solution containing 300 g/l of sodium sulfate and200 g/l of sulfuric acid served as electrolyte. It was circulated at arate of about 600 l/h from a circulation reservoir via a heat exchangerand through the cell by pumping (batch operation). Electrolysisoperation was maintained for 5000 hours, with only the water which hadevaporated or been decomposed being replaced. In steady-state operation,a concentration of 170-190 g/l of sodium peroxodisulfate was establishedat a steady-state temperature of about 35° C. The total voltage onstart-up was 50 V. The mean cell voltage changed as follows over thecourse of continuous operation: Operating time of 5 h 50 h 500 h 5000 hMean cell voltage 4.95 V 4.60 V 4.35 V 4.18 V

After 5000 hours of operation, the electrodes were removed and theweight loss was determined. The mean decrease in the silicon electrodethickness was calculated therefrom as an average of 3 μm. The thicknessof the silicon cathode thus decreases by only about 10 μm per year.

Example 2

The dependence of the current yield on the final concentration of sodiumperoxodisulfate (NaPS) achieved was determined by means of the undividedelectrolysis cell from example 1 under the same electrolysis conditions(current density, temperature, batch operation, electrolytecomposition). The following results were obtained: Final concentrationof NaPS in g/l 25 50 75 100 125 150 Current yield of NaPS 84 77 64 50 4034 formation in %

At the favorable cell voltage of about 4.2 V established after aprolonged period of operation, the specific electric energy consumptionwas 1.23 kWh/kg for a final concentration of 50 g/l; for a finalconcentration of 100 g/l of NaPS, it was still 1.89 kWh/kg despite thefact that the current yield had dropped to 50%.

Example 3

The same undivided electrolysis cell as in examples 1 and 2 was equippedwith a PVC gauze resting on the cathodes of the bipolar electrode platesand the boundary cathode; this gauze could be pressed onto the surfaceby means of a plastic spacer. Electrolysis was again carried out underthe same electrolysis conditions as in example 2. The following currentyields, based on the final NaPS concentration achieved, were obtained.Final concentration of NaPS in g/l 50 75 100 125 150 175 200 Currentyield of NaPS 84 77 73 68 61 54 49 formation in %

Even in the concentration range from 100 to 200 g/l, relativelyfavorable current yields were obtained and these were an average ofabout 20% higher than without shielding of the cathode surfaces.However, the cell voltages were about 0.8 V higher due to the additionalresistance of the gauze shielding. Nevertheless, a very favorablespecific electric energy consumption of about 1.85 kWh/kg was stillobtained at, for example, a final NaPS concentration of 150 g/l.

Example 4

The nine bipolar electrodes and the two monopolar boundary electrodes ofthe undivided electrolysis cell used in examples 1 to 3 were used in adivided bipolar cell. Cation-exchange membranes which were fixed on bothsides by means of anode and cathode spacers made of plastic were usedfor separating anolyte and catholyte. The anode and cathode spacesbounded by sealing frames had a thickness of 2-3 mm each. Anolyte andcatholyte were circulated in separate circuits through a heat exchanger.500 g/l of sulfuric acid served as catholyte. The anolyte once againconsisted of an aqueous solution containing 200 g/l of sulfuric acid and300 g/l of sodium sulfate. To avoid an excessively large decrease in thesodium sulfate concentration due to both consumption to formperoxodisulfate and the transport of Na⁺ ions through thecation-exchange membrane into the catholyte at the desired high finalNaPS concentrations, a further 100 g/l of sodium sulfate were dissolvedin the anolyte during the electrolysis (i.e. a total of 400 g/l ofsodium sulfate). The anodic and cathodic current densities were each setto 0.5 A/cm².

Under otherwise comparable electrolysis conditions, the followingcurrent yields were obtained for various final NaPS concentrations:

at a final NaPS concentration of 200 g/l, a current yield of 86%

at a final NaPS concentration of 300 g/l, a current yield of 82%

at a final NaPS concentration of 400 g/l, a current yield of 74%

The mean cell voltages were in the range from 5.5 to 6 V. At the finalconcentration of 400 g/l, a still very low specific electric energyconsumption of about 1.8 kWh/kg could thus be achieved.

1-7. (canceled)
 8. A process for preparing peroxodisulfuric acid or asalt thereof by performing electrolysis of an aqueous solutions of atleast one of sulfuric acid or a metal sulfate at diamond-coatedelectrodes without addition of a promoter, wherein bipolar siliconelectrodes which are coated on one side with doped diamond and whoseuncoated silicon rear side serves as a cathode.
 9. The process of claim8, wherein the electrolysis is carried out in undivided electrolysiscells.
 10. The process of claim 8, wherein the electrolysis is carriedout in electrolysis cells which are divided by at least one of anion-exchange membrane or a porous diaphragm.
 11. The process of claim 8,wherein a diamond-coated anode composed of a valve metal and providedwith a power supply lead is used as a boundary anode.
 12. The process ofclaim 9, wherein a diamond-coated anode composed of a valve metal, e.g.niobium, and provided with a power supply lead is used as a boundaryanode.
 13. The process of claim 10, wherein a diamond-coated anodecomposed of a valve metal, e.g. niobium, and provided with a powersupply lead is used as a boundary anode.
 14. The process of claim 8,wherein stainless steel, Hastelloy, platinum, impregnated graphite orsilicon which has been metallized on one side is used for the boundarycathode provided with a power supply lead.
 15. The process of claim 9,wherein stainless steel, Hastelloy, platinum, impregnated graphite orsilicon which has been metallized on one side is used for the boundarycathode provided with a power supply lead.
 16. The process of claim 10,wherein stainless steel, Hastelloy, platinum, impregnated graphite orsilicon which has been metallized on one side is used for the boundarycathode provided with a power supply lead.
 17. The process of claim 8,wherein a plurality of electrode stacks comprising bipolar electrodesand boundary electrodes with power supply lead are connectedelectrically in parallel within an electrolysis cell.
 18. A bipolarundivided or divided electrolysis cell comprising bipolar electrodescoated with diamond on one side.
 19. The process of claim 11, whereinthe valve metal is niobium.